We hope to throw light on periodic mechanisms underlying cyclic changes of behavior, mood, thinking and metabolism of manic depressives, catatonic-schizophrenics and various other periodic disorders such as periodic neutropenia and intermittent hydroarthrosis; also diurnal fluctuations--morning and evening changes--that occur in many normals. In man and animals we hope to throw light on periodic mechanisms in general and on the 24-hour clock in particular. Our major project will concern: (1) study of differences in 24- hour clocks of 10 species of animals--correlation with their various biological backgrounds--whether "dark" active, "light" active, "crepuscular" active, "arrhythmic" active; hibernators or non- hibernators; wild or domesticated; (2) study of differences in reactions of these animals to heavy water and (3) a study of inherent reactions of these animals to light and darkness using heavy water and the 24-hour clock as tools. Research on experimental production of behavioral cycles in rats and squirrel monkeys will be continued; also, we will continue collecting case histories of patients with various forms of periodic illnesses and then study them on basis of our knowledge obtained from experimental production and control of periodic manifestations in animals. Work on the book "The 24-Hour Clock of Animals and Man"--a summary of research carried on during the past 15 years is being continued. Also in preparation for publication is a book with Mr. P. D. Malone, chief artist at The Lahey Clinic in Boston, entitled "Experimental Surgery of the Rat," describing most operations that have been done on rats along with a text describing methods of determining success of the various operations.